Scientists may have solved a 60-year-old mystery by revealing that quasars are energetic objects powered by a predatory supermassive black holes and can outshine trillions of stars combined—formed when galaxies collide and merge.
The results suggest that the Milky Way could host its own quasar when it collides with the neighboring Andromeda galaxy in several billion years.
Scientists have previously tracked the bright, energetic emission of quasars in regions at the hearts of galaxies that span roughly the width of the Solar System—meaning that quasars must come from incredibly compact objects. A leading theory suggests that quasars are supermassive black holes that heat vast amounts of surrounding gas, releasing vast amounts of radiation before the material falls onto the surface of the black hole.
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Since their discovery six decades ago, quasars have puzzled scientists — largely because it’s unclear how supermassive black holes can attract enough raw material to produce such powerful emissions. Although supermassive black holes are at the centers of most galaxies, the gas needed to power quasars tends to orbit the galaxy’s outskirts. Thus, there must be some delivery service that directs the gas to the hearts of galaxies.
New research has now been published in the journal Monthly Notices of the Royal Astronomical Society uses deep imaging observations from the Isaac Newton Telescope in Spain’s Canary Islands to finally solve this puzzle.
“To understand how quasars are ignited, we need to determine how gas can fall into the center of galaxies at a high enough speed,” said the leader of the study. Clive Tadhunter, a professor in the Department of Physics and Astronomy at the University of Sheffield in the United Kingdom, told Live Science via email. “One idea is that the necessary radial infall is caused by collisions between galaxies, the associated gravitational forces of which can disturb the gas from its normal circular orbits.”
By comparing 48 nearby quasar-hosting galaxies to 100 non-quasar galaxies, the researchers discovered the presence of distorted structures at the edges of quasar-hosting galaxies. These structures also indicate a past or ongoing collision and merger with another galaxy, Tadhunter said.
“We found a high number of such structures in quasar-hosted galaxies, three times higher than a carefully matched control sample of non-quasar galaxies imaged using the same methods,” Tadhunter said. “This provides strong evidence that quasars are indeed triggered by galaxy collisions.”
The team’s research is not the first time that quasars have been linked to galaxy mergers. Tadhunter noted, however, that attempts to test this hypothesis by looking for distorted structures in the outer parts of galaxies typical of such collisions had previously proved inconclusive.
“Some studies have revealed the expected structures, while others have not,” he continued. “We believe that much of the past confusion in this field is due to the fact that many of the previous imaging studies did not have sufficient depth to detect the sometimes faint distortional structures in the outer parts of galaxies that host quasars.”
Quasars can have a profound effect on the evolution of the galaxies in which they reside; a better understanding of how quasars ignite could help scientists refine their models of galaxy evolution and the evolution of the universe as a whole.
“It’s important to understand how, when and where quasars are triggered because, once triggered, the enormous radiation output produced by a quasar can have significant, devastating effects on the surrounding galaxy,” Tadhunter said. “For example, radiation pressure can squeeze out any remaining gas in a galaxy system. Because gas is needed to form new stars, it cut off any future star formation activity, effectively the death throes of the galaxy.”
Tadhunter also pointed out that understanding the relationship between galactic collisions and quasars is crucial to determining the future of our own corner of space.
“The nearest large galaxy, the Andromeda spiral, is coming straight at us at high speed and will collide and merge with the Milky Way in about 5 billion years,” he said. “When this happens, the quasar is likely to be triggered, with gas falling into the center of the remnant system.”
The team plans to continue this research by examining other quasars at a wider range of distances that have been discovered using other methods to see if they share the same characteristics that link them to galactic collisions.
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